Fluid streams such as liquid flows and gaseous flows (e.g. air flows) often carry solid particulates and undesirable liquid droplets that are often undesirable contaminants entrained in the fluid stream. Filters are commonly employed to remove some or all of the particulates from the fluid stream. Often filters are characterized as depth filters wherein particulates filter throughout the depth of the filter media, or as surface filters, whereby particulates are trapped along the surface of the filter media.
Filter media including fine fibers formed using an electrostatic spinning process is also known. Such prior art includes Filter Material Construction and Method, U.S. Pat. No. 5,672,399; Cellulosic/Polyamide Composite, U.S. Patent Publication No. 2007/0163217; Filtration Medias, Fine Fibers Under 100 Nanometers, And Methods, U.S. Patent Publication No. 2009/0199717; Integrated Nanofiber Filter Media, U.S. Patent Publication No. 2009/0266759; Filter Media Having Bi-Component Nanofiber Layer, U.S. Provisional Patent Application No. 61,047,455; Expanded Composite Filter Media Including Nanofiber Matrix and Method, U.S. Provisional Patent Application No. 61/308,488; and Compressed Nanofiber Composite Media, U.S. Provisional Patent Application No. 61/330,462, the entire disclosures of which are incorporated herein by reference thereto.
Often, such fine fibers have been employed in surface loading applications. There is publication proposing use of gradient depth media in the form of a single pleated sheet of filter media comprising alternative layers of scrim and nanofibers, such as in U.S. Pat. No. 6,521,321 to Kahlbaugh et al. While the '321 patent to Kahlbaugh suggests that the possibility of an application to liquid filtration, it acknowledges through examples and in description that it is primarily focused upon air or gas filtration (e.g. cabin air, engine air, tobacco smoke). Further, the depth filtration appears limited to the depth of the sheet itself as it is often discussed in association with a pleated configuration. Further, it is believed that previously fine fibers (and particularly electrospun fibers) have generally not been applied to liquid applications due to the momentum of liquids and viscosity to tend to pull particulates through such a fine fiber layer. Instead it is believed that such fine fibers have typically been employed in air filtration applications.